1. Field of the Invention
The invention concerns a wear monitor for controlling the wear and tear of a component that moves with a rubbing motion while pressed against a motionless surface, in particular for controlling the wear and tear of rider rings, piston rings and suchlike, with a sensor that monitors the thickness of the moving component at least indirectly and communicates with an evaluation unit.
2. The Prior Art
Arrangements of the above-named kind are known in all sorts of connections and are mostly applied for the purpose of being able to keep the eroding component in use for example as a guiding element, an insulating element, a braking element or suchlike as long as possible (in other words as far as an at least factually objective given wear threshold), in order to keep the downtime usually necessary for replacing the eroded component as short as possible in relation to rest of the serviceable life of the equipment. In the majority of the cases of application employed as examples, a direct, for example visual, check of the wear and tear is actually only possible otherwise by stopping the equipment and usually dismantling it at least in part, which procedure of inspection to be carried out for the sake of caution however often involves unacceptable downtimes.
In connection with the wear monitoring of piston rings and suchlike, which is in particular demand, the underlying problem can be expressed for example in relation to piston compressors as follows: especially when the piston is not set vertically and in larger machinery, the weight of the piston is usually supported at present by the cylinder liner, via slideways. For this purpose, the cylinder is usually fitted with rider rings, which in lubricated compressors consist of metal materials, preferably soft metal alloys, or of plastics, which slide on a film of grease. In the case of non-lubricated reciprocating piston compressors, practically exclusive use is made of rider rings made of non-metallic materials (especially plastics) with particularly favourable tribological properties, although these are of course subject to a certain degree of wear and tear. In order to avoid damage to the cylinder liner or to the pistons themselves, these rider rings may only be used until they have a given residual thickness, at which point they must be replaced. In order to determine the moment when replacement is necessary, it is therefore important to know the residual thickness of the rider rings at any given time. In order to avoid switching the compressor off unnecessarily, it is desirable to be able to determine the wear and tear of the rider rings without dismantling the compressor cylinder, preferably while the compressor is running.
As the measurement of the residual thickness of the component subject to wear and tear--as for example the above-described rider rings inside the cylinder of a piston compressor--is, as indicated, only possible with difficulty and in any case against considerable expenditure, because of the conditions under which the relative equipment is normally used, a variety of procedures and systems have been known to be used to date for determining indirectly the condition of wear and tear of the component in question. Thus for example, in the case of the reciprocating piston compressor described above, an apparatus known from U.S. Pat. No. 4,987,774 A makes an indirect determination of the residual thickness of the rider rings based on the vertical position of the piston rod. The measurement of the drop of the piston rod is determined, for example, using an inductive transducer situated in a position that is easily accessible from outside, e.g. on the side of the piston rod packing facing the crank-case. The problem arising here is of course that the distance measured between the sensor and the piston rod is influenced by countless factors. In addition to the thickness of the rider ring that is actually of interest, the vertical position of the cross-head of the compressor, the deflection of the piston rod, the vertical play of the piston in the cylinder and the distance between the point of measurement and the rider ring also play a role that is ultimately difficult to consider accurately. Moreover, the result of the measurement is further influenced by tolerances, geometric relationships, momentary crank angle positions as well as deformations as a consequence of forces exerted. In order to eliminate these disturbing influences, provision was already made in the said known apparatus for determining the piston rod drop at regular intervals with a suitably selected crank angle, which eliminates many of the effects mentioned that distort the measurement result, so that the wear and tear of the rider rings can be determined so as to achieve a relatively favourable approximation. For this purpose, it is of course necessary to synchronise the measurement with the machine speed, for which the signal from an additional transducer applied on the disc flywheel or on another suitable position on the crank mechanism is used.
In order to avoid the additional installation expense for a speed sensor or angular encoder of this kind, it has also been proposed to acquire the course over time of the piston rod drop over the piston stroke and to deduce the wear and tear of the rider rings by suitable mathematical evaluation by omitting a speed-synchronous signal.
Both aforementioned methods embody a further disadvantage of the inductive measurement of the piston rod drop in the form of its sensitivity to the magnetic properties of the piston rod, because the same is often provided with a layer whose purpose is to inhibit wear and tear. These layers sometimes have irregular wall thicknesses and often also comprise diamagnetic properties, so that the determination of the piston rod drop in this way may also include principal errors.
Furthermore, according to EP 538 771 A1, for example, a procedure for diagnosing the wear and tear in moving machine parts was proposed, whereby, in connection with the measurement of the wear and tear taking place in rider rings of piston compressors functioning horizontally, the drop of the piston rod is determined by means of a pneumatic transducer functioning on the basis of the nozzle-baffle principle, thus avoiding the above-mentioned difficulties with the inductive measuring methods. The other described disadvantages of this kind of determination of the wear and tear, however, remain also in this case.
In order to further improve the aforementioned methods for monitoring the wear and tear of the rider rings in piston compressors in particular, systems have also been proposed in which the drop of the piston rod is measured simultaneously in two different positions, whereby the influence of the vertical position of the piston cross-head can be eliminated with knowledge of the current crank angle and suitable evaluation. Nevertheless, it has proved to be impossible to demonstrate these theoretical advantages in practice, as the measurement errors in the two measurements interfere with each other adversely.
The task of the invention is to improve a wear monitor of the kind described above in such a way that the mentioned disadvantages of the known arrangements and procedures are avoided and in particular that it is possible to obtain a significant statement of the determination of the residual thickness of the moving components, such as the said rider rings of piston compressors, in a simple way.